Invasive Fish Species

Many aquarists prefer to release their fish into the wild rather than euthanize them when they have grown bored of their aquarium or must get rid of the fish due to some other reason. If you release fish species that are not native to your region, you can however cause sever problems for the local wild life. This is not only true for fish, but for plants, invertebrates and other living creatures as well.

According to the National Invasive Species Information Center (NISIC), an invasive species is defined as a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions.

Invasive species that have not caused any apparent harm to the native ecosystem are sometimes instead referred to as introduced species, but the terms can be used interchangeably. Other commonly used terms include non-native species, non-indigenous species, exotic species, alien species, and transplants.

Ecosystems are complicated and intricate, and there is no way of telling how a certain species will affect the native flora and fauna. It is therefore very important for the responsible aquarist to refrain from releasing any species into the wild. Even if you find it unlikely that your particular aquarium fish would cause any harm to your regional wildlife, you should still refrain from releasing it since there is no way of knowing how your species will react to their new environment. You aquarium inhabitants can also carry malicious microbes that will spread to the wild. Even if your fish do not survive in its new home, the microbes might find new hosts to infect – hosts that have never encountered this particular microbe before and have no way of combating it. This means that even if your aquarium fish is naturally occurring in your region, you should refrain from releasing it if it has been kept together with creatures from other parts of the world that might have infected it with micro organism.

You might think that as long as you release one single fish into the wild there is no way for it to establish a breeding population, but this is only true if there are no other aquarists in your area. If one aquarist release a female fish, while another release a male fish, there is always a theoretical possibility of establishing a breeding population. You should also keep in mind that closely related species can interbreed and form hybrids, and fish hybrids can be fertile and able to reproduce.

Living creatures have of course spread from region to region ever since the first living organisms developed on Earth, and ecosystems have always been faced with new members and forced to adjust in various ways. When we discus invasive species, we sometimes tend to view the world’s ecosystems as static phenomena, but this is naturally not true. We should however keep in mind that human travel has grown to amazing proportions during the last 500 years and this adds an entirely new pressure on the ecosystems. A gradual change that would have required many centuries or even millennia can now be achieved within a year when an irresponsible aquarist have emptied his or her tropical aquarium into a lake or stream. The ecosystem will eventually adapt to the new species, but we might not like the new ecosystem. In sensitive ecosystems, such as the one found in Hawaii, we can for instance see how invasive species cause native and endemic species to vanish.

When a breeding population has become established and we notice the first signs of trouble in the ecosystem, it can be very hard to do anything about the invasive species without harming the rest of the ecosystem. Releasing your fish, plants or invertebrates into the wild thinking “if any problems do occurs, the authorities can always kill the species” is therefore not a good idea. There are many examples of how invasive species have proven very difficult to eradicate.

An invasive species can affect the ecosystem in several different ways. Some aquarium species might for instance have no natural predators in the region where they have been released. This means that they can multiply rapidly without being combated by natural enemies. Other aquarium species will happily gulp down the native flora or fauna – a native flora and fauna that might have no ways of protecting itself from this new and unknown enemy. A very successful predator can also harm other predators, since they will compete for prey. So called “genetic pollution” is also a potential problem. The invasive species might be able to interbreed with native species and thereby change the gene pool.

As mentioned above, invasive species can also introduce micro organisms to the new environment. A species that has gradually developed in the presence of certain fungi, bacteria, virus, internal parasites etcetera will have developed a natural resilience against these certain species due to natural selection. When a new aquarium species is introduced to a region, it can carry within itself micro organism that will pose a serious treat to the native flora or fauna since the native species have no natural resilience against the new health problems.

One method that has proven successful in some cases where an invasive species has disrupted an ecosystem is to introduce yet another invasive species to the ecosystem. This is naturally a very unpredictable way of combating invasive species, since we do not know how the ecosystem will react to the second invasive species. It is however often the best way of combating a problem causing invasive species without harming the native flora or fauna. The invasive Water hyacinth (Eichhornia crassipes) from South America has caused a lot of problem in Lake Victoria on the African continent, but is today combated with the aid of yet another South American species – a weevil that loves to eat Water hyacinth. Before the weevil was introduced to Lake Victoria, a series of tests were carried out to ensure that the weevil would not feed on native flora or local crops.

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